Process for bleaching wood pulp



Nw.. 393g, J, SCHUER 78,649

PROCESS FOR BLEACHING WOOD'PULP Fl'ed Nov. lO, 1938 BNVENTOR ATTORNEYPatented Nov.. 7, 1939 UNITED STATES PATENT OFFICE PROCESS FOR BLEACHINGWOOD PULP Application November 10, 1938, Serial No. 239,763

comme.

This invention relates to a process for bleaching wood pulp.

Bleaching processes involving the following separate stages have beenproposed; chlorination, wherein the pulp in aqueous suspension istreated with elemental chlorine to chlorinate the lignin and otherimpurities, making them in part soluble in water or in alkali; causticextraction, wherein a part of the chlorinated impurities are dissolvedfrom the fibers by digestion with a caustic solution; and hypochloritebleaches in one or more stages, wherein the pulp suspension is subjectedto an alkaline hypochlorite solution to oxidlze the remaining insolubleimpurities, making them soluble in water or in alkali.

In bleaching pulps containing considerable proportions of diillcultlysoluble impurities, such as kraft pulp or any under-cooked pulps, it hasbeen suggested to apply alkaline hypochlorite bleaches in at least twostages after the initial chlorine bleach in order to solubilizesubstantially completely the greater amount of insoluble impuritiespresent in such pulps. Since the iirst hypochlorite treatment partiallyoxidizes the residual lignin chloride and other insoluble color bodiesremaining in the fibers after chlorination and alkali extraction and theimpurities are thereby rendered partially soluble in water, and solubleto a greater extent in alkali, it has been" proposed to remove theseoxidized impurities by a caustic extraction stage prior to subjectingthe pulp to further hypochlorite treatment.

They addition of alkali/during the hypochlorite treatment so asf'toavoid the vdevelopment of VV acidity` which-would cause deterioration ofthe pulp being treated has been mentioned in the-1.

hypochlorite treatment, g, This caustic soda,v -it isV stated, may beadded from time to time 'tomamv'tain the desired alkaline conditionsthroughout sired bleach. The exhaustion'of the alkalinity of the bleachsolution may be determined by adding phenolphthalein to a sample portionof the solution; if no color develops this indicates that the alkalinityhas been exhausted.

I have made the surprising discovery that bleaching of pulp can beaccomplished efficiently and a pulp of high strength, purity andwhiteness obtained by maintaining alkaline conditions within a pH rangefrom about 8.3 to 10.0, preferably about 8.5, during one or more of thebleach steps to which the pulp is subjected, in which steps the pulp istreated to remove such impurities as require an oxidation treatment (e.g. hypochlorite treatment) for their removal and, when the bleach in thestep in question is completely or almost completely consumed, addingalkali to increase the pH to a value not greater than 13.0, preferablyabout 12.0. One method oi determining exhaustion of hypochlorite bleachis to titrate a sample to which potassium iodide has been added with astandard thiosulfate solution. From this maybe calculated the actualconcentra-` tion of available chlorine in the solution and in turn thedegree of exhaustion of the hypochlorite. In actual bleaching practice,however, a simpler method is used, namely, a starch potassium iodidepaper which is discolored by chlorine in solution.

An experienced operator can ascertain the approximate extent ofexhaustion of the bleach solution -by observing the rate of colordevelopmention the test paper.

It is important to note that in the bleaching :step in which myinvention is employed, i. e.` prior toA exhaustion or substantiallycomplete exhaustion ofthe bleach. .in the step in question, analkalinity of from about 8.3 to 10.0 is maintained and thereafter alkaliis added to raise the pH to higher levels. With some pulps 1% sodiumhydroxide on the basis of bone dry pulp is suilcient for such alkaliaddition. With other pulps larger quantities up to 5% and 10% causticsoda may be required, depending upon the type of pulp and the bleachedproduct desired. Thel caustic soda serves to extract dyes which, ifallowed to remain, detract from the whiteness of the finished A product.I have found that if a concentration of alkali suflicient to performthis extraction is present throughout the bleaching step, bleaching isnot properly conducted, as above pointed out,

in that the alkali slows down the bleaching to such an extent as toaffect adversely the quality of the pulp.

In carrying out the bleaching of pulp in accordance with this invention,an aqueous suspension of the wood pulp, which may have undergoneprevious treatments such as chlorination and caustic extraction, istreated, in order to oxidize and thereby solubilize the difilcultlysoluble lignin and chloro-lignin bodies and other colored impurities sunremaining in the puip, with a. bleachv solution such as calciumhypochlorite or sodium hypochlorite, chlorine or hypochlorous essarydependingy on the pulp being bleached and conditions under which thebleaching is carried out. As hypochlorite is the most common reagentused at the present time in the paper industry the invention willhereinafter be described in connection with the use of this reagent; itbe understood, however, that the invention is not confined to its usebut other bleach solutions such as those mentioned may be used in lieuof hypochlorite.

Under the conditions of pH dening the invention, the hypochlorite bleachsolution exerts its maximum oxidizing effect on the impurities retainedby the fibers without attacking the fibers themselves appreciably. Whenthe hypochlorite has become substantially consumed, sumcient alkali isadded to the mixture to dissolve from the fibers the impurities oxidizedby the hypochlorite. A pH of 12 has been found advantageous for thispurpose. After solution of impurities by the alkali, the pulp may bewashed and subjected to further bleaching operations.

It is important that the solution be maintained alkaline during thehypochlorite treatment because the presence of hydroxyl ion exerts acatalytic eect promoting oxidation by the hypochlorite. Moreover, inacid solution the hypochlorous acid and hypochlorite present in thesolution attack the cellulose fibers rapidly, resulting in a great lossin ber strength. It is important that the upper pH limit of 10.0 beobserved because at a pH above 10.0 the presence of the hydroxyl ion hasso great an adverse effect on the equilibrium of the reactions involvedthat the rate of oxidation of impurities by the hypochlorite isconsiderably reduced, with a resultant attack upon the cellulose fibersby the alkaline solution during the necessarily longl time of contactand the loss in vfiber quill-1t? and strength caused thereby.Thelnaintenance of a pH range of 8.5 to 9 during the bleach isparticularly advantageous because. under such conditions, I have foundthe hypochlorite solution exerts its maximum oxidizing effect on theimpurities present with the least possible amount of injury to the pulpfibers.

The addition of alkali directly to the mixture of pulp and substantiallyexhausted hypochlorite solution for the purpose of dissolving oxidizedimpurities eliminates a water-washing operation found necessary in priorart processes, and eilects a saving in equipment and handling costs oversuch prior art processes.

One example of the practicer of the process of this invention ishereinbelow described in connection with the accompanying drawingforming n a part of this specincation. This drawing showsdiagrammatically, for purposes of exempliilcation, a preferred klayoutused in the bleaching of spruce kraft pulp. This layout was designed fora four-stage treatment as follows:

(1) chlorination:

(3) Caustic extraction;

(3) Hypochlorite bleach combined with caustic extraction;

(4) A second hypochlorite bleach.

Referring to the drawing, the unbleached pulp from the storage tank Iwas led into the chlorinator 2 in the form of an aqueous suspension of3.5% consistency. The pulp was chlorinated at 65 F. with a weightedamount of gaseous chlorine yrepresenting 60% of the chlorine demand ofthe pulp. The time of chlorination was 45 minutes. The pulp was thenthoroughly ywashed in the vacuum washer I.

'Ihe pulp was brought to a consistency of 15% and was digested at atemperature of 110 F. with about 1% of sodium hydroxide based on the dryweight oi' the pulp. After 45 minutes the maximum solubilizlng efect onthe chlorinated impurities had been reached and the pulp was washed inthe vacuum washer 5 until no longer alkaline to phenolphthalein.

In vthe bleaching tank i the pulp at a consistency of about was treatedwith calcium hypochlorite in accordance with my invention in amountequivalent to about 30% of the chlorine demand of the pulp, the solutionduring this treatment having a ing out this example it was foundunnecessary to add alkali to maintain the pH` constant. The temperaturewas maintained at about 95 F. After 30 minutes about 90% of thehypochlorite had been consumed. At this point caustic soda was added inamount equal to about 1% NaOH based on the dry pulp and the temperaturewas raised to 110 F. The caustic extraction was allowed to proceed for45 minutes and the pulp was then thoroughly washed in the vacuum washer1.

In the bleaching tank 8 the washed pulp at a consistency of about 10%was treated for about two hours at a temperature of about 95 F. with an`amount of calcium hypochlorite equivalent to about 10% of the chlorinedemand of the pulp. The pH was maintained at about 8.5. After thoroughwashing in the vacuum washer 9 the bleached pulp was stored in the tanki0.

The above treatment resulted in a bleached pulp of high white colorretaining the desirable strength characteristics oi the unbleached pulp.For example, kraft pulp bleached by the above four-stage processembodying my invention, when tested in the standard General Electricbrightness tester, showed a more brilliant white color than the samepulp bleached by a fourstage process involving chlorination, causticextraction, and two hypochlorite bleaches in which during thehypochlorite bleaches usual pH conditions were maintained and not theconditions of' this invention. Moreover, in addition to more effectivebleaching, as shown by better color, the tearing resistance of papermade from the above pulp bleached according to my invention was morethan 16% higher than the tearing resistance of paper made from the otherpulp referred to above.

I'he time 'of treatment, temperature, pulp consistency, and otherconditions mentioned in the above example will vary as the invention isapplied to the bleaching of other pulps.

In the caustic extraction stage following chlorination, the amount ofcaustic soda normallyrequired for solution of the alkali-solubleresidues varies from 0.5% to 1.5% of the dry pulp. With easy bleachingsulilte pulp 0.5% is usually all that is necessary, whereashard-bleaching kraft pulp may consume the entire 1.5% added in theextraction stage. Increased caustic concentrapH of about 8.5. Incarrytion and temperatures in the caustic extraction stage introduceproportional solution of hemicellulose, with increase in thelalpha-cellulose content of the pulp. The amount of alkali added todissolve oxidized impurities when the hypochlorite has been consumed inthe rst hypochlorite stage may be as high as 5% to 10% of the dry pulpin some cases, depending on the characteristics of the pulp itself andthe type of product desired.

During the hypochlorite oxidation, acidic products are sometimes formed.It is necessary in such cases to add small amounts of caustic from timeto time during either or both of the hypochlorite treatments in order tomaintain the desired pH in the solution during oxidation. In a greatmany cases, however, it is unnecessary to add any caustic at all duringthe hypochlorite oxidation, the alkalinity of the calcium hypochlorite,together with the alkali carry-over from the previous causticextraction, being suicient to maintain the proper pH until the bleach isexhausted.

Substantially equivalent reagents may be substituted for thosespecically mentioned above. Thus, caustic potash, sodium carbonate,lime, or other strong alkali may be used in place of caustic soda foralkali extraction.

In applying this invention to the bleaching of pulps containing moreeasily solubilized impurities, one or more of the stages enumeratedabove may be omitted. Thus, in some cases the bleaching process may becarried out by treating the pulp initially with hypochlorite solution,with addition of caustic when the hypochlorite is consumed, inaccordance with the invention, the pulp thereafter being washed andsubjected to a second hypochlorite treatment; or the initial treatmentmay consist of chlorination, followed by one or more hypochloritestages, one or more of which is combined with caustic extraction inaccordance with the invention.

When it is desired to bleach a very impure pulp to a high degree ofwhiteness, additional stages may be included in the bleaching process.Thus a second chlorination may precede either the rst or the secondhypochlorite stages described in the example. In some cases thebleaching process may include three hypochlorite stages, the first twocombined with caustic extraction in accordance with the invention. Forsome pulps it may be advantageous to substitute for the causticextraction stage following chlorination a combined hypochloriteoxidation and caustic extraction stage.

It is to be understood that this invention is not restricted to thepresent disclosure otherwise than defined by the appended claims.

I claim:

1. A process for bleaching wood pulp which comprises mixing an aqueoussuspension of the pulp with a solution of a hypochlorite, maintainingthe alkalinity of the mixture at a pH between 8.3 and `10 until thehypochlorite has been substantially completely consumed and thereafterincreasing the pH of said mixture.

2. A process for bleaching wood pulp which comprises mixing an aqueoussuspension of the pulp with a solution of a hypochlorite of a metal fromthe group comprising alkaline earth and alkali metals, maintaining thepH of the mixture at a value between 8.5 and 9 until the hypochloritehas been substantially completely con- 3. In a process for bleachingwood pulp, the step which comprises contacting an aqueous suspension ofthe pulp with a solution of a hypochlorite of a metal from the groupcomprising alkaline earth and alkali metals, maintaining the mixture ata pH of about 8.3 to 10 until the hypochlorite has been substantiallycompletely consumed and thereafter increasing the alkalinity of themixture by the addition of caustic soda to dissolve the alkali-solubleimpurities.

4. A process for bleaching wood pulp requiring a plurality of bleachingtreatments, which comprises treating an aqueous suspension of the pulpwith hypochlorite of a metal from the group comprising alkaline earthand alkali metas to oxidize impurities in the pulp, maintaining a pH of8.3 to 10 in the digestion mixture until about 90% of the hypochloritehas been consumed, thereafter treating said mixture with caustic soda inamount suiiicient to raise the pH to effect dissolution of thealkali-soluble oxidation products, washing the pulp to removesubstantially all the caustic soda, and treating the pulp with a furtherquantity of hypochlorite solution while maintaining the pH above 8.3.

5. A multi-stage process for bleaching kraft pulp comprising thefollowing separate stages of treatment: (1) chlorinating an aqueoussuspension of the pulp with elemental chlorine; (2) digesting the pulpsuspension with a solution of an alkali to dissolve the alkali-solublechlorinated impurities; (3) mixing the pulp suspension with a. solutionof a hypochlorite, maintaining a pH between 8.3 and 10 in the mixtureuntil the hypochlorite has been substantially completely consumed, andthereafter adding an alkali to said mixture to increase the pH todissolve the impuriu ties rendered alkali-soluble by hypochloriteoxidation; and (4) mixing the pulp suspension with a further quantity ofhypochlorite to oxidize the residual impurities in the pulp, whilemaintaining a pH between 8.3 and 10 in the mixture.

6. A multi-stage process for bleaching kraft pulp comprising thefollowing separate stages of treatment: (1) chlorinating an aqueoussuspension of pulp with an amount of gaseous chlorine representing 60%of the chlorine demand of the pulp; (2) digesting the pulp suspensionwith sodium hydroxide in the amount of about 1% based on the dry weightof the pulp to dissolve the alkali-soluble chlorinated impurities; (3)mixing the pulp suspension with a solution of calcium hypochlorite in anamount equivalent to at least of the chlorine demand of the pulp,maintaining a pH between 8.5 and 9 in the mixture until about 90% of thehypochlorite has been consumed, and thereafter adding sodium hydroxideto said mixture in the amount of about 1% based on the dry pulp toincrease the pH to about 12 and to dissolve the impurities renderedalkalisoluble by hypochlorite oxidation; and (4) mixing VAthe pulpsuspension with an amount of hypochlo- 7. rIhe step in the process forbleaching pulp v which comprises treating an aqueous suspension of thepulp with a chlorine bleach solution, maintaining during said treatmenta pH of from about 8.3 to 10 until the bleach has been substantiallycompletely exhausted and thereafter increasing the alkalinity of themixture.

J OHN SCHUBER.

